A prototype expert system to diagnose faults in a telecommunications network was developed for a large retail, hotel and brewery company. Principal objectives met were that it should run on a PC, should be usable by unskilled persons, would find all straightforward faults and would lead the operator through all the steps needed to activate the relevant maintenance contractor and record the fault in a log. The topology of the network, together with names and telephone numbers of appropriate people to call up at the network nodes, were all stored in files, thus enabling changes to be made without interfering with the knowledge base. A survey is given of applications of expert systems in related fields and the special features of expert systems together with the reasons for choosing to develop one rather than to write a conventional program are discussed. The system was developed for the SAVOIR expert system shell, and details of the knowledge base are given as well as some comments on the suitability of that shell for this task.